While electronic devices are coming out with reduced dimension and enhanced performance, high frequency noises radiated from a signal wire and a power wire (hereinafter referred to generically as “signal line”) are becoming a major problem. These noises can be suppressed most simply by inserting the signal line through a closed magnetic path core called “bead core” shaped toroidal or cylindrical. Practically, the signal line is often inserted through the core with one turn therearound (one turn insertion), and in some cases, the core may be provided with a plurality of through-holes, through which the signal line is inserted for a plurality of turns therearound.
A bead type noise filter, which leverages the noise suppressing method described above, will hereinafter be explained with reference to FIG. 1. FIG. 1 shows a closed path magnetic core (bead core) 1 formed of a soft magnetic material having a high resistivity. The magnetic core 1 is shaped cylindrical, has a through-hole 1a along its center axis, and is attached on a signal line 2 such that the signal line 2 is inserted through the through-hole 1a, thereby functioning as a bead type noise filter. In this connection, the magnetic core 1 may alternatively be provided with a plurality of through-holes 1a (refer to FIG. 2).
In order to suppress unwanted radiation noises of several ten to several hundred MHz, the magnetic core 1 has been conventionally formed of Ni—Zn ferrite which has a high resistivity (102 to 105 Ωm) so as to duly work in such a high frequency band. Also, the magnetic core 1 is required to have a high resistivity so as to keep away from troubles even when the signal line 2 inserted through the through-hole 1a is not provided with an insulating coat thus possibly making direct contact therewith, which is another reason for using Ni—Zn for the magnetic core 1. The usage of Ni—Zn ferrite containing en expensive material of Ni pushes up the cost of the bead type noise filter in spite of its simple structure.
On the other hand, conventional general Mn—Zn ferrite is an inexpensive soft ferrite but has a resistivity measuring as low as 10−1 to 100 Ωm. Consequently, the magnetic core 1 formed of Mn—Zn ferrite cannot be used in a high frequency band, because eddy current loss increases strikingly in a frequency band lower than a signal frequency band where noises are required to be suppressed. Also, there is another problem with Mn—Zn ferrite that the signal line 2 without an insulating coat must not be disposed in direct contact with the magnetic core 1 when inserted through the through-hole 1a. 
Mg—Zn ferrite is another inexpensive soft ferrite. Mg—Zn ferrite, however, is inferior to other soft ferrites in soft magnetic properties, such as initial permeability and saturation magnetic flux density, and so the magnetic core 1, when formed of Mg—Zn ferrite, must have an increased dimension in order to achieve equivalent characteristics as a bead type noise filter. Especially, when used to suppress noises in the signal line 2 (here, particularly a power wire) where ripple surge and surge noise present a problem, the magnetic core 1 must have its dimension further increased for prevention of magnetic saturation. Consequently, Mg—Zn ferrite is not used as a bead type noise filter.
Referring to FIG. 3, an equivalent circuit of a bead type noise filter is represented as a parallel circuit consisting of a series circuit of an inductance L component and a resistance R component, and a capacitance component C. Hereinafter, the inductance L component, the resistance R component, and the capacitance C component may be referred to simply as L component, R component, and C component, respectively, as appropriate.
In FIG. 3, in a frequency band of a signal to be transmitted (signal frequency band), there is a relation of: L>>R, where L=value of the L component, and R=value of the R component, each representing impedance |Z| value. Hence, the series circuit of the L and R components in the bead type noise filter functions as inductance (almost L component only), and the signal to be transmitted does not incur a loss caused due to the R component. Thus, the bead type noise filter, in conjunction with the C component at the signal line, constitutes a low-pass filter in terms of a circuit. But, since the value of the L component is small, and since a cutoff frequency is higher than a frequency band of the signal to be transmitted, the transmission loss of the signal can be ignored.
In a frequency band higher than the frequency band of a signal to be transmitted, there is a relation of L<<R, so the series circuit of the L and R components functions as resistance (almost R component only), and absorbs noises as heat. This contributes to effectively suppressing, in particular, unwanted radiation noises.
The impedance |Z| of the bead type noise filter can be split into a reactance X component (hereinafter referred to simply as X component, as appropriate) and a resistance R component as follows:|Z|=√{square root over ( )}(X2+R2)  Formula (1)
When, an AC magnetic field is applied to the magnet core of the bead type noise filter, its complex permeability μ can be represented as follows:μ=√{square root over ( )}(μ′2+μ″2)  Formula (2)where μ′ is a real part, and μ″ is an imaginary part. The X component of the bead type noise filter (the component may be “L component”) is generated by the real part μ′ while the R component thereof is generated by the imaginary part μ″.
Consequently, the X component is dominant in a frequency band of the signal to be transmitted, which means that the bead type noise filter, in conjunction with the C component at the signal line, functions as a low-pass filter, and blocks passage of noise components superposed on the signal. However, the noise components blocked and prevented from passing may possibly affect other circuits. On the other hand, in a high frequency band where radiation noises are generated, the R component is dominant and converts noise components including the radiation noises into thermal energy, which constitutes a noise removing factor. Noises can be removed more safely and reliably when converted into thermal energy than when blocked by a low-pass filter.
A frequency at which the X and R components have an equal value, namely an X-R cross point, is a marginal frequency where dominance changes over between the X component to reflect noises and the R component to convert noises into thermal energy. Generally, the more the R component to convert noises into thermal energy is, the better it is in terms of removing noises, so a lower X-R cross point is more preferred if impedance characteristic is equal.
And, if the X component is large and the R component is small in a high frequency band, an LC resonance with a large Q (index to show inductance performance) is generated by the capacitance C component at the signal line, whereby a digital signal inputted may suffer waveform distortion, such as ringing, depending on a circuit to which the bead type noise filter is connected. Consequently, in a high frequency band, the smaller the X component is, the better it is.
In connection with the above-described relation between a frequency (band) and X and R components, the X-R cross point of a bead type noise filter constituted by a magnetic core formed of the aforementioned Ni—Zn ferrite is found approximately at 10 MHz which belongs to a high frequency band, and its reactance X component still keeps increasing in a high frequency band. So, if such a bead type noise filter using Ni—Zn ferrite is attached on an input signal line of a high impedance digital circuit having a capacitance of several pF, such as a C-MOS inverter, digital signals inputted suffer waveform distortion, such as ringing, undershoot, and overshoot. This is caused due to the generation of the above-described LC resonance having a comparatively large Q. For the reason of this waveform distortion as well as the aforementioned expensiveness of Ni—Zn ferrite, a bead type noise filter with a magnetic core formed of non-Ni—Zn ferrite has been demanded. Especially, a bead type noise filter which has a lower X-R cross point than one with a magnetic core formed of Ni—Zn ferrite, and which suppresses noises mainly by means of an R component in a safe and reliable manner is demanded.
The present invention has been made in light of the demand described above, and it is an object of the present invention to provide an inexpensive bead type noise filter which suppresses noises without distorting the waveform of a transmission signal, such as a digital signal, in order to deal with higher frequency and digitalization of signals in recent electronic devices.
Specifically, the object of the present invention (according to Claim 1) is to provide a bead type noise filter with a magnetic core formed of inexpensive Mn—Zn ferrite which has its resistivity significantly increased so as to achieve soft magnetic properties comparable to those of Ni—Zn ferrite in a high frequency band for the purpose of suppressing radiation noises of several 10 to several 100 MHz, also to provide an inexpensive bead type noise filter which allows a signal wire or power wire without an insulating coat to be inserted therethrough in a direct contact manner if needed. Further, the object of the present invention (according to Claims 2 and 3) is to eventually provide a high-performance bead type noise filter which, on top of accomplishing the above object, suppresses noises mainly by means of an R component in a safe and reliable manner so as not to distort the waveform of transmission signals.